Protein kinase JAK (Janus kinase) is a protein-tyrosine phosphorylation enzyme present in the cytoplasm that regulates functions involved in the growth and survival of cells in the lymphohematopoietic system. JAK is activated by stimulation via a cytokine receptor, and induces the phosphorylation of tyrosine residues of STAT (signal transducers actuators of transcription) protein. The phosphorylated STATs dimerize and translocate from the cytoplasm to the nucleus where they bind to specific DNA sequences leading to the transcriptional activation of genes (Gene, 285, 1-24, 2002).
The JAK family is known to be composed of four members: JAK1, JAK2, JAK 3 and Tyk2. While Jak1, Jak2 and Tyk2 are expressed relatively ubiquitously, the expression of Jak3 is localized. JAK3 is constantly expressed in NK cells, thymic cells, mast cells, platelet cells etc., whereas in T cells and B cells its expression is induced following the activation of the cells. JAK3 is specifically associated with the γc chain of the interleukin (IL)-2 receptor and is activated by each cytokine stimulation via each receptor of IL-2, IL-4, IL-7, IL-9, IL-13, IL-15 and IL-21 (Curr. Pharm. Design., 10, 1767-1784, 2004). It is demonstrated that JAK3 is also involved in IL-2 production from T cells and T cell activation by associating with the T cell receptor/CD3 complex (J. Immunol., 163, 5411-5417, 1999; J. Biol. Chem., 276, 25378-25385, 2001). Furthermore, in some patients with severe combined immunodeficiency disease (SCID), the reduced expression of the JAK3 protein due to JAK3 gene mutation can be noted, and in patients with X-linked severe combined immunodeficiency disease (XSCID), gene defect in the γc chain has been reported suggesting that the blockage of JAK3-related signal transduction may inhibit the immune system (Nature, 377, 65-68, 1995; Science 266, 1042-1045, 1994). Furthermore, it is reported that in JAK3-deficient mice, the onset of streptozotocin-induced autoimmune diabetes mellitus can be suppressed (Curr. Pharm. Design., 10, 1767-1784, 2004).
Since JAK3 is expressed in many lymphatic cells, involved in the activation and propagation of T cells, and has been implicated in autoimmune diseases of model animals, as described above, it is expected to provide a selective target for drug discovery and the development of an agent for specifically inhibiting JAK3 has been sought after.
On the other hand, a study on gene-deficient mice suggested that JAK3 is involved in degranulation and release of chemical mediators induced by IgE/antigen stimulation, JAK3 is also promising as a target for drug discovery intended to inhibit allergic reactions associated with mast cells (Biochem. Biophys. Res. Commun., 257, 807-813, 1999).
Accordingly, it is expected that inhibition of JAK3 could lead to the prevention and treatment of rejection and graft versus host disease (GvHD) in organ transplantation, rheumatoid arthritis, multiple sclerosis, systemic lupus erythematosus, Sjögren syndrome, Behcet's disease, type I diabetes mellitus, autoimmune thyroiditis, idiopathic thrombocytopenic purpura, ulcerative colitis, Crohn's disease, asthma, allergic rhinitis, atopic dermatitis, contact dermatitis, urticaria, eczema, psoriasis, allergic conjunctivitis, uveitis, cancer, leukemia and the like.
Under these circumstances, low molecular weight inhibitors for JAK3 intended for pharmaceuticals have been reported. For example, a pyrrolo-pyrimidine derivative (WO2000/142246; Bioorg. Med. Chem. Lett., 17, 1250-1253, 2007), a pyrrolo-pyridine derivative (WO2007/007919), an indolone derivative (Bioorg. Med. Chem. Lett., 13, 3105-3110, 2003), a purine derivative (WO2006/108103), a benzoxazole derivative (WO2008/031594 pamphlet), a quinazoline derivative (WO2000/010981), a quinoline derivative (WO2005/075429), a highly-fused ring compound (WO2007/145957; Bioorg. Med. Chem. Lett., 17, 326-31, 2007), a pyrimidine derivative (WO2008/009458; WO2006/133426) and the like.
Also, there is a report on a pyridine derivative having a JAK3 inhibitory activity (WO2007/062459), which describes the following the general formula:
[see the description for definition of symbols]
However, the pyridine derivative described in this reference is different from the pyridine derivative of the present invention in the type and in the position of bonding of functional group, and the 50% inhibition concentration for the JAK3 inhibitory activity described in the Example is about 20 μM (see the description of the present invention on pages 49 to 50). There are no further reports on JAK3 inhibitors of the pyridine type, and thus pyridine derivatives having an excellent JAK3 inhibitory activity are still in great need.